Micro-channel evaporator with frost detection and control

ABSTRACT

A refrigerant vapor compression system includes an evaporator having a plurality of longitudinally extending, flattened heat exchange tubes disposed in parallel, spaced relationship. Each of the heat exchange tubes has a flattened cross-section and defining a plurality of discrete, longitudinally extending refrigerant flow passages. One or more frost detection sensor(s) is/are installed in operative association with the evaporator for detecting a presence of frost/ice formation on at one of the flattened heat exchange tubes and associated heat transfer fins. A defrost system is provided and operatively associated with the evaporator heat exchanger A controller, operatively coupled to the frost detection sensor(s) and to the defrost system, selectively activates the defrost system to initiate a defrost cycle of the evaporator in response to the signal indicative of the presence of frost formation on the flattened heat exchange tubes and heat transfer fins.

FIELD OF THE INVENTION

This invention relates generally to evaporator heat exchangers and, moreparticularly, to providing for improved control of frost accumulation onthe external surfaces of evaporator heat exchangers having a pluralityof parallel, flattened heat exchange tubes.

BACKGROUND OF THE INVENTION

Air conditioners and heat pumps employing refrigerant vapor compressioncycles are commonly used for cooling or cooling/heating air supplied toa climate controlled comfort zone within a residence, office building,hospital, school, restaurant or other facility. Refrigerant vaporcompression systems are also commonly used for cooling air, or othersecondary media such as water or glycol solution, to provide arefrigerated environment for food items and beverage products withindisplay cases, bottle coolers or other similar equipment insupermarkets, convenience stores, groceries, cafeterias, restaurants andother food service establishments.

Conventionally, these refrigerant vapor compression systems include acompressor, a condenser, an expansion device, and an evaporator seriallyconnected in refrigerant flow communication. The aforementioned basicrefrigerant vapor compression system components are interconnected byrefrigerant lines in a closed refrigerant circuit and arranged in accordwith the vapor compression cycle employed. The expansion device,commonly an expansion valve or a fixed-bore metering device, such as anorifice or a capillary tube, is disposed in the refrigerant line at alocation in the refrigerant circuit upstream, with respect torefrigerant flow, of the evaporator and downstream of the condenser. Theexpansion device operates to expand the liquid refrigerant passingthrough the refrigerant line connecting the condenser to the evaporatorto a lower pressure and temperature. The refrigerant vapor compressionsystem may be charged with any of a variety of refrigerants, including,for example, R-12, R-22, R-134a, R-404A, R-410A, R-407C, R717, R744 orother compressible fluid.

In some refrigerant vapor compression systems, the evaporator is aparallel tube heat exchanger having a plurality of flattened, typicallyrectangular or oval in cross-section, multi-channel heat exchange tubesextending longitudinally in parallel, spaced relationship between afirst generally vertically extending header or manifold and a secondgenerally vertically extending header or manifold, one of which servesas an inlet header/manifold. The inlet header receives the refrigerantflow from the refrigerant circuit and distributes the refrigerant flowamongst the plurality of parallel flow paths through the heat exchanger.The other header serves to collect the refrigerant flow as it leaves therespective flow paths and to direct the collected flow back to therefrigerant line for return to the compressor, in a single pass heatexchanger, or to a downstream bank of parallel heat exchange tubes, in amulti-pass heat exchanger. In the latter case, this header is anintermediate manifold or a manifold chamber and serves as an inletheader to the next downstream bank of parallel heat transfer tubes.

Each heat exchange tube generally has a plurality of flow channelsextending longitudinally in parallel relationship the entire length ofthe tube, each channel providing a relatively small cross-sectional arearefrigerant flow path. Thus, a heat exchanger with multi-channel tubesextending in parallel relationship between the inlet and outlet headersof the heat exchanger will have a relatively large number of smallcross-sectional area refrigerant flow paths extending between the twoheaders. Sometimes, such multi-channel heat exchanger constructions arereferred to as microchannel or minichannel heat exchangers as well.Commonly, for evaporator applications, the heat exchanger generallyincludes heat transfer fins positioned between heat transfer tubes forheat transfer enhancement, structural rigidity and heat exchanger designcompactness. The heat transfer tubes and fins are permanently attachedto each other (as well as to the manifolds) during a furnace brazeoperation. The fins may have flat, wavy, corrugated or louvered designand typically form triangular, rectangular, offset or trapezoidalairflow passages.

When a heat exchanger is used as an evaporator in a refrigerant vaporcompression system, moisture in the air flowing through the evaporatorand over the external surfaces of the refrigerant conveying heatexchange tubes and associated heat transfer fins of the heat exchangercondenses out of the air and collects on the external surfaces of thoseheat exchange tubes and heat transfer fins. Depending upon operatingconditions, the moisture condensing out of the air may accumulate on theexterior surfaces of the heat exchange tubes and heat transfer fins ofthe evaporator and form frost or ice. As the accumulation of frost orice on the heat exchange tubes and heat transfer fins increases andbuilds up closing the airflow passages between the fins and the tubes,particularly in the regions where the fins contact the tube, heattransfer between the refrigerant within the tubes and the air passingover the tubes decreases, as a result of the increase in thermalconduction resistance caused by the frost or ice layer. Additionally, ifthe frost build-up between the fins becomes excessive, the air-sidepressure drop through the evaporator increases, resulting in a decreasein airflow delivered by an air-moving device, thereby furtherdeteriorating the overall performance of the evaporator heat exchanger.

Further, unlike the larger diameter round heat exchange tubes withrelatively large spaces between the tubes, commonly used in conventionalrefrigerant evaporators, flattened, multi-channel tubes defining aplurality of small cross-sectional area flow passages are subject andmore susceptible to damage from the accumulation of frost or ice on theexternal surfaces of the heat exchange tubes and associated heattransfer fins. For the conventional round tube and plate fin heatexchanger constructions condensing water tends to more readily drain offthe heat exchange tubes and along heat transfer fins. However, onflattened tubes, the condensing water tends to accumulate rather thandrain off the tubes. Consequently, the accumulating water, unlessremoved from the tube, will alternately freeze, at certain operatingconditions, forming frost or ice and then melt (fully or partially)during a defrost cycle. Since water expands upon freezing, repeatedfreezing and thawing of the accumulated condensate, particularly in theconfined spaces between the heat transfer fins and the flattened heatexchange tubes (e.g., in the region where the fins contact the flattenedtubes), can damage the heat exchanger by deforming or cracking the tubeand causing separation of the fins from the tubes. Furthermore, duringsequential defrost cycles, more ice may accumulate on external surfacesof the heat exchange tubes and heat transfer fins of the evaporator heatexchanger and may even completely block airflow passages, forcing theevaporator to run outside of a specified operational envelope (in termsof suction pressure) and compromising refrigerant system reliability orcausing nuisance shutdowns, both of which are obviously highlyundesirable events.

In refrigerant vapor compression systems having conventional finnedround tube and plate fin evaporators, it is common practice to defrostthe evaporator either periodically for a timed interval or on demand asthe need to defrost is sensed. For example, U.S. Pat. No. 6,205,800discloses a demand defrost method for defrosting the evaporator of arefrigerated display case, wherein a defrost cycle is initiated when thedifference between the sensed air temperature within the case and thesensed refrigerant temperature equals or exceeds a defrost threshold.The refrigerant temperature sensor is mounted externally on therefrigerant inlet tube to the evaporator or other location in theevaporator coil or internally within the refrigerant inlet tube.Examples of frost sensors disclosed in the art for use in connectionwith evaporator defrost on demand control systems include thermistors,such as disclosed in U.S. Pat. No. 4,305,259; capacitive sensor plates,such as disclosed in U.S. Pat. No. 4,347,709; air velocity sensors, suchas disclosed in U.S. Pat. No. 4,831,833; fiber optic sensors, such asdisclosed in U.S. Pat. No. 4,860,551; and heat flow sensors, such asU.S. Pat. No. 6,467,282.

SUMMARY OF THE INVENTION

A refrigerant vapor compression system includes a refrigerant flowcircuit having a refrigerant compressor, a condenser, an expansiondevise and an evaporator connected serially in refrigerant flowcommunication. The evaporator has a plurality of longitudinallyextending, flattened heat exchange tubes disposed in parallel, spacedrelationship. Each of the heat exchange tubes has a flattenedcross-section and may define a plurality of discrete, longitudinallyextending refrigerant flow passages. At least one frost detection sensoris installed in operative association with the evaporator for detectinga presence of frost or ice formation on at least one of the flattenedheat exchange tubes or heat transfer fins and generates a signalindicative of the presence of frost or ice formation on that flattenedheat exchange tubes and heat transfer fins. A defrost system isoperatively associated with the evaporator. A controller, operativelycoupled to the frost/ice detection sensor and to the defrost system,selectively activates the defrost system to initiate a defrost cycle ofthe evaporator in response to the signal indicative of the presence offrost or ice formation on at least one of the flattened heat exchangetubes and heat transfer fins. The frost/ice detection sensor may be asingle sensor installed at a single location on the heat exchanger or aplurality of frost detection sensors installed at different locations onthe heat exchanger.

In an embodiment, the frost detection sensor may be a sensor mounted onan exterior surface of one of the flattened heat exchange tubes or heattransfer fins. In an embodiment, a plurality of frost detection sensorsmay be mounted on the exterior surfaces of a number of differentflattened heat exchange tubes, heat transfer fins or a combination ofthereof. In an embodiment, the defrost system may be an electric defrostheater system. In an embodiment, the defrost system may be a hot gasdefrost system for selectively passing at least a portion of refrigerantvapor from the compressor through the heat exchange tubes of theevaporator.

The heat exchanger may have flattened heat exchange tubes having aflattened generally rectangular or oval cross-section, each of which maydefine multiple internal fluid flow passages having a flow area of acircular cross-section or a non-circular cross-section. The heatexchanger may also include a plurality of fins extending betweenadjacent flattened heat exchange tubes. The fins may be a plurality ofgenerally vertical fins extending between adjacent heat exchange tubesor a plurality of fins may comprise serpentine-like fins extendingbetween adjacent heat exchange tubes and may be of a louvered, wavy,offset strip or flat plate configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed description of the invention, reference willbe made to and is to be read in connection with the accompanyingdrawing, where:

FIG. 1 is a schematic diagram of a refrigerant vapor compression systemincorporating a multi-channel heat exchanger as an evaporator;

FIG. 2 is a perspective view of an exemplary embodiment of an evaporatorheat exchanger equipped with a defrost sensor;

FIG. 3 is a schematic diagram of a refrigerant vapor compression systemincorporating a multi-channel heat exchanger evaporator with a defrostsensor and an associated electric defrost heater; and

FIG. 4 is a schematic diagram of a refrigerant vapor compression systemincorporating a multi-channel heat exchanger evaporator with a defrostsensor and an associated hot gas defrost system.

DETAILED DESCRIPTION OF THE INVENTION

The heat exchanger of the invention will be described herein in use asan evaporator in connection with a simplified air conditioning cyclerefrigerant vapor compression system 100 as depicted schematically inFIG. 1. Although the exemplary refrigerant vapor compression cycleillustrated in FIG. 1 is a simplified air conditioning cycle, it is tobe understood that the heat exchanger of the invention may be employedin refrigerant vapor compression systems of various designs, including,without limitation, heat pump cycles, economized cycles, cycles withtandem components such as compressors and heat exchangers, chillercycles, cycles with reheat and many other cycles including variousoptions and features.

The refrigerant vapor compression system 100 includes a compressor 105,a condenser 110, an expansion device 120, and the heat exchanger 10,functioning as an evaporator, connected in a closed loop refrigerantcircuit by refrigerant lines 102, 104 and 106. The compressor 105circulates hot, high pressure refrigerant vapor through dischargerefrigerant line 102 into the inlet header of the condenser 110, andthence through the heat exchange tubes of the condenser 110 wherein thehot refrigerant vapor is desuperheated, condensed to a liquid andtypically subcooled as it passes in heat exchange relationship with acooling fluid, such as ambient air, which is passed over the heatexchange tubes by the condenser fan 115. Although the heat exchanger 110is referred to as a condenser throughout the text, as known to a personordinarily skilled in the art, a predominantly two-phase subcriticalcondenser heat exchanger becomes a single-phase gas cooler, intranscritical applications. Both subcritical and transcriticalapplications of the heat exchanger 10 can equally benefit from theinvention described herein.

The high pressure, liquid refrigerant leaves the condenser 110 andthence passes through the liquid refrigerant line 104 to the evaporatorheat exchanger 10, traversing the expansion device 120 wherein therefrigerant is expanded to a lower pressure and temperature to form arefrigerant liquid/vapor mixture. The now lower pressure and lowertemperature, expanded refrigerant thence passes through the heatexchange tubes 40 of the evaporator heat exchanger 10 wherein therefrigerant is evaporated and typically superheated as it passes in heatexchange relationship with air to be cooled and, in many cases,dehumidified, which is passed over the heat exchange tubes 40 andassociated heat transfer fins 50 by the evaporator fan 15. Therefrigerant leaves the evaporator heat exchanger, predominantly in avapor thermodynamic state, and passes through the suction refrigerantline 106 to return to the compressor 105 through the suction port.

As the airflow traversing the evaporator heat exchanger 10 passes overthe heat exchange tubes 40 and heat transfer fins 50 in heat exchangerelationship with the refrigerant flowing through the heat exchangetubes 40, the air is cooled and the moisture in the air flowing throughthe evaporator beat exchanger 10 and over the external surface of therefrigerant conveying tubes 40 and heat transfer fins 50 of theevaporator heat exchanger 10 condenses out of the air and collects onthe external surfaces of the heat exchange tubes 40 and heat transferfins 50. A drain pan 45 is provided beneath the evaporator heatexchanger 10 for collecting that condensate which drains from theexternal surface of the heat exchange tubes 40 and heat transfer fins50.

The parallel flow heat exchanger 10 includes a plurality of heatexchange tubes 40 of generally flattened cross-section, which arearranged in parallel relationship in a generally vertical array. In theexemplary embodiment of the heat exchanger 10 depicted in FIG. 2, eachof the heat exchange tubes 40 extends in a generally horizontaldirection along its longitudinal axis between a generally verticallyextending first header 20 and a generally vertically extending secondheader 30, thereby providing a plurality of parallel refrigerant flowpaths between the two headers. Although the refrigerant headers 20 and30 are shown of a cylindrical configuration, the may be of arectangular, half of a cylinder or any other shape, as well as have asingle chamber or multi-chamber design, depending on the refrigerantpath arrangement. Each heat exchange tube 40 has a first end mounted tothe first header 20, a second end mounted to the second header 30, andat least one flow channel 42 extending longitudinally, i.e. parallel tothe longitudinal axis of the tube for the entire length of the tube,thereby providing a flow path in refrigerant flow communication betweenthe first header 20 and the second header 30. The heat exchangerrefrigerant pass arrangement may be of a multi-pass configuration, suchas depicted in FIG. 2, or of a single-pass configuration, depending onparticular application requirements.

Each heat exchange tube 40 comprises an elongated tubular memberextending along its longitudinal axis and having a generally flattenedcross-section, for example, a rectangular cross-section or ovalcross-section. The flattened tubular member has an upper wall 46 and alower wall 48 and defines the at least one longitudinally extendinginternal fluid flow passage 42. The at least one internal fluid flowpassage 42 may be subdivided into a plurality of parallel, independentinternal fluid flow passages 42 which extend longitudinally parallel tothe longitudinal axis of the heat exchange tube 40 in a side-by-sidearray, thereby providing a multi-channel heat exchange tube. Eachflattened heat exchange tube 40 has a leading edge 41 which facesupstream, with respect to the airflow through the heat exchanger 10, anda trailing edge 43 which faces downstream, with respect to the airflowthrough the heat exchanger 10.

Each flattened multi-channel tube 40 may have a width as measured alonga transverse axis extending from the leading edge 41 to the trailingedge 43 of, for example, fifty millimeters or less, typically from tento thirty millimeters, and a height of about two millimeters or less, ascompared to conventional prior art round heat exchange tubes having adiameter of ½ inch, ⅜ inch or 7 mm. The heat exchange tubes 40 are shownin the accompanying drawings, for ease and clarity of illustration, ashaving ten internal channels 42 defining flow paths having a rectangularcross-section. However, it is to be understood that in applications,each multi-channel heat exchange tube 40 may typically have from aboutten to about twenty internal flow channels 42. Generally, each internalflow channel 42 will have a hydraulic diameter, defined as four timesthe cross-sectional flow area divided by the “wetted” perimeter, in therange generally from about 200 microns to about 3 millimeters. Althoughdepicted as having a rectangular cross-section in the drawings, theinternal flow channels 42 may have a circular, triangular, oval ortrapezoidal cross-section, or any other desired non-circularcross-section. Also, heat transfer tubes 40 may have other internal heattransfer enhancement elements, such as mixers and boundary layerdestructors.

As in conventional practice, to improve heat transfer between the airflowing through the heat exchanger 10 over the external surfaces of theflattened heat transfer tubes 40 and the refrigerant flowing through theinternal parallel flow channels 42 of the heat transfer tubes 40, theheat exchanger 10 includes a plurality of external heat transfer fins 50extending between each set of the parallel-arrayed tubes 40. The heattransfer fins are brazed or otherwise securely attached to the externalsurfaces of the upper and lower walls of the respective tubular membersof adjacent heat exchange tubes 40 to establish heat transfer contact,by heat conduction, between the heat transfer fins 50 and the externalsurface of the flat heat exchange tubes 40. Thus, the external surfacesof the heat exchange tubes 40 and the surfaces of the heat transfer fins50 together form the external heat transfer surface that participates inheat transfer interaction between the air flowing through the heatexchanger 10 and refrigerant flowing through the internal channels 42.The external heat transfer fins 50 also provide for structural rigidityof the heat exchanger 10 and quite often assist in air flow redirectionto improve heat transfer characteristics.

In the exemplary embodiment of the heat exchanger 10 depicted in FIG. 2,the heat transfer fins 50 constitute segments of a fin plate formed as aserpentine series of generally V-shaped or generally U-shaped segmentsand are disposed in heat transfer contact with the both lower externalsurface of the lower wall 48 of one heat exchange tube 40 and the upperexternal surface of the upper wall 46 of the adjacent heat exchange tube40 next therebelow. Alternatively, the fins may constitute a pluralityof plates disposed in parallel, spaced relationship and extendinggenerally vertically between the heat transfer tubes 40. It is to beunderstood that other fin configurations, such as, for example,generally corrugated, wavy, louvered or offset fins forming triangular,rectangular, or trapezoidal airflow passages may be used in the heatexchanger of the invention.

As noted hereinbefore, a heat exchanger used as an evaporator inrefrigerant vapor compression system, such as for example, but notlimited to, an air conditioning or refrigeration system, are subject towater condensing out of the air flow passing through the evaporator andcollecting on the external surfaces of the heat exchange tubes and heattransfer fins of the heat exchanger. Under certain operating conditionstypically experienced over the course of normal operation, thecondensate will freeze forming frost or ice on the upper and lowerexterior surfaces 46, 48 of the flatted heat exchange tubes 40 and onthe heat transfer fins 50, particularly in the region where the heattransfer fins 50 contact the upper and lower exterior surfaces of theheat exchange tubes 40.

To detect frost or ice formation on the heat exchanger 10, at least onefrost detection sensor 60 is installed in operative association with theheat exchanger 10. In the exemplary embodiment of the heat exchanger 10depicted in FIG. 2, a frost detection sensor 60 is mounted to theexterior surface of one of the heat exchange tubes 40. However, it is tobe understood that a frost detection sensor 60 could instead be mountedon the surface of one of the heat transfer fins 50. Additionally, it isto be understood that a plurality of frost detection sensors 60 may beinstalled on the heat exchanger 10, including the locations on the heatexchange tubes and/or the heat transfer fins, with each defrostdetection sensor 60 mounted at a different location within the heatexchanger 10. For example, a frost detection sensor may be installed inthat region of the heat exchanger 10 where frost/ice tends to accumulatefirst and most excessively or a frost detection sensor 60 may beinstalled at each of a number of different locations throughout the heatexchanger 10 whereat frost/ice tends to accumulate. The precise locationor locations at which frost detection sensors should be installed in aparticular heat exchanger is a matter of choice within the skill of theordinary practitioner in the art. The selection of the type of frostdetection sensor 60 to be used is also within the skill of the ordinarypractitioner in the art, and not limiting of the invention. The frostdetection sensor 60 may be a heat flux sensor, a strain gauge sensor orany other type of sensor capable of detecting the formation of frost onthe exterior surface of the heat exchange tubes 40.

The frost detection sensor 60 is operatively coupled to a controller 80and provides a signal to the controller 80 indicative of the formationof frost on the exterior surface of the heat exchange tube 40 with whichthe sensor 60 is associated. In an embodiment, the frost detectionsensor 60 provides a signal to the controller 80 indicative of theactual degree of frost formation on the exterior surface of the heatexchange tube 40 with which the sensor 60 is associated. The controller80 processes the signal received from the frost detection sensor(s) 60and determines whether or not the amount of frost formation indicated isexcessive. If so, the controller 80 then initiates a defrost cycle tomelt the frost formed on the evaporator heat exchanger 10.

Referring now to FIG. 3, in the refrigerant vapor compression system 100therein depicted, an electric defrost system is operatively associatedwith the evaporator heat exchanger 10. The electric defrost systemcomprises an electric defrost heater that includes at least one electricheating element 65 disposed at or slightly upstream, with respect to airflow, of the air inlet to the evaporator heat exchanger 10. In theexemplary embodiment depicted in FIG. 3, a plurality of electric heatingelements 65 are provided, one electric heating element 65 associatedwith each heat exchange 40. When the controller 80 determines that adefrost cycle is to be initiated, the controller 80 energizes theelectric heating elements 65. The electric heating elements 65 operateas in conventional practice to heat the air entering the evaporator heatexchanger 10 sufficiently above 0° C. to cause the frost formed withinthe heat exchanger to melt as the heated air flows through the heatexchanger. Also, the electric heating elements 65 would heat theexternal surfaces of the heat exchanger 10, which would assist inmelting the ice as well. Alternatively, if the safety and isolationmeans are installed in place, at least some of the heat exchange tubes40 or heat transfer fins 50 can be used as electric heating elements 65.After a pre-selected time interval, the controller 80 will de-energizethe electric heating elements 65 thereby ending the defrost cycle.

In the exemplary embodiment of the refrigerant vapor compression system100 depicted in FIG. 4, a hot gas defrost system is operativelyassociated with the evaporator heat exchanger 10. The hot gas defrostsystem includes a hot gas defrost line 70 and a flow control valve 90operatively disposed in the hot gas defrost line 70. The hot gas defrostline 70 has an inlet opening in refrigerant flow communication with anintermediate pressure stage of the compressor 105 and an outlet openingin refrigerant flow communication with refrigerant line 104 at alocation upstream, with respect to refrigerant flow, of the evaporatorheat exchanger 10 and downstream, with respect to refrigerant flow, ofthe expansion device 120. Thus, the hot gas defrost line 70 provides arefrigerant flow path from an intermediate pressure stage of thecompressor 105 to the refrigerant inlet line to the evaporator heatexchanger 10. The flow control valve 90 may be selectively positionedbetween a closed position whereat the flow control valve 90 closes thehot gas defrost line 70 to refrigerant flow therethrough and an openposition whereat the flow control valve 90 opens the gas defrost line 70to refrigerant flow therethrough. In an embodiment, the flow controlvalve 90 may be a solenoid electrically operated flow control valve.Further, the flow control valve 90 can be of a modulating or pulsatingtype, respectively modulating or cycling between the closed and openpositions. Also, if the compressor 105 is not equipped with theintermediate pressure port, a discharge refrigerant vapor can beutilized instead for the defrost purposes, which is obviously within thescope of the invention.

The flow control valve 90 is operatively coupled to the controller 80.As noted before, the controller 80 processes the signal received fromthe frost detection sensor(s) 60 and determines whether or not theamount of frost/ice formation indicated is excessive. If so, thecontroller 80 then initiates a defrost cycle to melt the frost/iceformed on the external surfaces of the evaporator heat exchanger 10 bysending a command signal to the flow control valve 90 causing the flowcontrol valve 90 to partially or fully open. With the flow control valve90 open, at least a portion of an intermediate pressure or dischargepressure refrigerant vapor passes from the compressor 105 through thehot gas defrost line 70 to enter the refrigerant line 104 and mix withthe expanded refrigerant vapor passing from the expansion device 120,thereby raising the temperature of the refrigerant vapor passing throughthe heat exchange tubes 40 of the evaporator heat exchanger 10. Thishigher temperature refrigerant vapor raises the temperature of thetubular elements defining the heat exchange tubes 40 as it traverses theflow passages 42 therethrough to a temperature sufficiently above 0° C.to cause the frost/ice formed within the heat exchanger 10 to melt asthe heated air flows through the evaporator heat exchanger. After apre-selected period of time, the controller 80 commands the flow controlvalve 90 to close, thereby preventing refrigerant vapor flowingtherethrough from the compressor 105 to the refrigerant line 104 andterminating the defrost cycle. Also, if desired, the refrigerant flowthrough the main refrigerant circuit could be completely blocked, whenthe defrost cycle is initiated. In this case, an additional flow controlvalve would be installed on the discharge line 102 and closed during thedefrost cycle by the controller 80.

As mentioned previously, if intermediate pressure vapor is not availableas a defrost medium, discharge pressure vapor may be used. In that case,the inlet of the hot gas defrost line 70 would be in refrigerant flowcommunication with the discharge pressure side of the compressor 105.The outlet of the hot gas defrost line 70 would again be in refrigerantflow communication with the refrigeration circuit at a locationupstream, with respect to refrigerant flow, of the evaporator 10 anddownstream, with respect to refrigerant flow, of the expansion device120. Furthermore, if the refrigerant system 100 is a heat pump,switching between heating and cooling modes of operation can be employedas the defrost means.

Also, it has to be understood that although the embodiments of theinvention are disclosed and would be most beneficial for application toevaporator heat exchangers with a horizontal orientation of the straightheat exchange tube array, the invention disclosed herein would also bebeneficial in application to evaporator heat exchangers with other heatexchange tube orientations and configurations, for example verticallyoriented heat exchange tubes or heat exchange tubes oriented at aninclination angle between 0 to 90 degrees with respect to the horizontalaxis.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be effected therein without departing from the spirit andscope of the invention as defined by the claims.

1. A refrigerant vapor compression system including a refrigerant flowcircuit comprising: a refrigerant compressor, a heat rejection heatexchanger, an expansion device and an evaporator connected serially inthe refrigerant flow circuit in refrigerant flow communication, saidevaporator having a plurality of longitudinally extending, flattenedheat exchange tubes disposed in parallel, spaced relationship, each ofsaid heat exchange tubes having flattened cross-section; at least onefrost detection sensor installed in operative association with saidevaporator for detecting a presence of frost or ice formation on theevaporator external surfaces and generating a signal indicative of thepresence of frost or ice formation at least at one location within theevaporator; a defrost system operatively associated with saidevaporator; and a controller operatively coupled to said at least onefrost detection sensor and to said defrost system, said controllerselectively activating said defrost system to initiate a defrost cycleof said evaporator in response to said signal indicative of the presenceof frost or ice formation at least at one location within theevaporator.
 2. A refrigerant vapor compression system as recited inclaim 1 wherein each of said flattened heat exchange tubes of saidevaporator defines a plurality of internal discrete, longitudinallyextending refrigerant flow passages.
 3. A refrigerant vapor compressionsystem as recited in claim 1 wherein said defrost system comprises anelectric heating system operatively associated with said evaporator. 4.A refrigerant vapor compression system as recited in claim 1 whereinsaid defrost system comprises a hot gas defrost system for selectivelypassing at least a portion of refrigerant vapor from said compressorthrough said heat exchange tubes of said evaporator.
 5. A refrigerantvapor compression system as recited in claim 4 wherein said hot gasdefrost system comprises: a hot gas defrost line having an inlet openingin refrigerant flow communication with an intermediate pressure stage ofsaid compressor and an outlet opening in refrigerant flow communicationwith the refrigeration cycle circuit at a location upstream, withrespect to refrigerant flow, of said evaporator and downstream, withrespect to refrigerant flow, of said expansion device; and a refrigerantflow control device disposed in said hot gas defrost line andoperatively coupled to said controller, said refrigerant flow controldevice being selectively positionable between a closed position and anopen position.
 6. A refrigerant vapor compression system as recited inclaim 5 wherein said refrigerant flow control device is of a modulationor pulsation type.
 7. A refrigerant vapor compression system as recitedin claim 4 wherein said hot gas defrost system comprises: a hot gasdefrost line having an inlet opening in refrigerant flow communicationwith a discharge pressure side of said compressor and an outlet openingin refrigerant flow communication with the refrigeration cycle circuitat a location upstream, with respect to refrigerant flow, of saidevaporator and downstream, with respect to refrigerant flow, of saidexpansion device; and a refrigerant flow control device disposed in saidhot gas defrost line and operatively coupled to said controller, saidrefrigerant flow control device being selectively positionable between aclosed position and an open position.
 8. A refrigerant vapor compressionsystem as recited in claim 7 wherein said refrigerant flow controldevice is of a modulation or pulsation type.
 9. A refrigerant vaporcompression system as recited in claim 1 wherein said refrigerant systemis a heat pump and said defrost system comprises switching betweenheating and cooling modes of operation.
 10. A refrigerant vaporcompression system as recited in claim 1 wherein said at least one frostdetection sensor comprises a frost detection sensor mounted on anexterior surface of one of said flattened heat exchange tubes.
 11. Arefrigerant vapor compression system as recited in claim 1 wherein saidat least one frost detection sensor comprises a frost detection sensormounted on a surface of one of the heat transfer fins positioned betweensaid heat exchange tubes and permanently attached to these heat exchangetubes.
 12. A refrigerant vapor compression system as recited in claim 1wherein said at least one frost detection sensor comprises a pluralityof frost detection sensors, each of said frost detection sensorsinstalled at a different location within said evaporator.
 13. Arefrigerant vapor compression system as recited in claim 1 wherein saidat least one frost detection sensor comprises a plurality of frostdetection sensors, each of said frost detection sensors mounted on anexterior surface of a different one of said flattened heat exchangetubes.
 14. A refrigerant vapor compression system as recited in claim 1wherein said at least one frost detection sensor comprises a pluralityof frost detection sensors, each of said defrost detection sensorsmounted on a surface of a different one of the heat transfer finspositioned between said heat exchange tubes and permanently attached tothese heat exchange tubes.
 15. A refrigerant vapor compression system asrecited in claim 1 wherein said flattened heat exchange tubes have aflattened rectangular or flattened oval cross-section.
 16. A refrigerantvapor compression system as recited in claim 2 wherein said plurality ofinternal discrete, longitudinally extending fluid flow passages have acircular cross-sectional flow area.
 17. A refrigerant vapor compressionsystem as recited in claim 2 wherein said plurality of internaldiscrete, longitudinally extending fluid flow passages have anon-circular cross-sectional flow area.
 18. A refrigerant vaporcompression system as recited in claim 1 further comprising a pluralityof heat transfer fins extending between adjacent flattened heat exchangetubes of said evaporator.
 19. A refrigerant vapor compression system asrecited in claim 18 wherein said plurality of heat transfer finscomprises a plurality of generally vertical fins extending betweenadjacent heat exchange tubes.
 20. A refrigerant vapor compression systemas recited in claim 18 wherein said plurality of heat transfer finscomprises serpentine-like fins extending between adjacent heat exchangetubes.
 21. A refrigerant vapor compression system as recited in claim 20wherein said a serpentine-like heat transfer fins extending betweenadjacent heat exchange tubes form one of generally triangular,rectangular or trapezoidal airflow passages.
 22. A refrigerant vaporcompression system as recited in claim 18 wherein said plurality of heattransfer fins are at least one of louvered, wavy, offset strip or flatplate configurations.
 23. A refrigerant vapor compression system asrecited in claim 1 wherein said evaporator has a first manifold and asecond manifold and said flattened heat exchange tubes extendlongitudinally between sad first and second manifolds in a single-passconfiguration.
 24. A refrigerant vapor compression system as recited inclaim 1 wherein said evaporator has a first manifold and a secondmanifold and said flattened heat exchange tubes extend longitudinallybetween sad first and second manifolds a multi-pass configuration.
 25. Arefrigerant vapor compression system as recited in claim 1 wherein saidflattened heat exchange tubes of said evaporator have a generallyhorizontal orientation.
 26. A refrigerant vapor compression system asrecited in claim 1 wherein said flattened heat exchange tubes of saidevaporator have a generally vertical orientation.
 27. A refrigerantvapor compression system as recited in claim 1 wherein said flattenedheat exchange tubes of said evaporator have an inclination angle between0 and 90 degrees, with respect to the horizontal axis.
 28. A refrigerantvapor compression system as recited in claim 1 wherein said evaporatorhas a generally vertical orientation.
 29. A refrigerant vaporcompression system as recited in claim 1 wherein said evaporator has aninclination angle between 0 and 90 degrees, with respect to thehorizontal axis.
 30. A refrigerant vapor compression system as recitedin claim 1 wherein said flattened heat exchange tubes of said evaporatorhave a generally straight configuration.